Saline resistant cementitious binders

ABSTRACT

SALINE CORROSION OF CONCRETE AT ELEVATED TEMPERATURES IS PREVENTED BY LIMITING THE SULFATE CONTENT OF THE CEMENT BINDER TO A LEVEL AT WHICH THE APATITE MATERIAL ELLESTADITE WILL NOT FORM.

Ab 112 EX IUD-O90 United States Patent Int. Cl. C04b 7/02 US. Cl. 106-89 4 Claims ABSTRACT OF THE DISCLOSURE Saline corrosion of concrete at elevated temperatures is prevented by limiting the sulfate content of the cement binder to a level at which the apatite mineral ellestadite will not form.

BACKGROUND OF THE INVENTION The detrimental effects on concrete of many salt solutions, especially those containing sulfate ion, are Well known. It is generally recognized that common, Portland cement-type concretes become subject to attack at concentrations above about 300 p.p.m. Above about 2000 p.p.m., sulfate attack becomes quite severe and special cement formulations are required. Choice of the proper type of cement for concrete structures exposed to sulfate attack is based primarily upon experience gained in practice.

Much of the sulfate-caused corrosion can be traced to formation and crystallization of sulfate-containing comice It is a further object of our invention to provide concrete formulations resistant to high concentrations of sulfate ion.

A specific object of our invention is to prevent the deterioration of concrete in contact with sulfate ions at temperatures above about 90 C.

DETAILED DESCRIPTION OF THE INVENTION We have discovered that concrete can be made resistant 10 to attack from saline solutions at elevated temperatures by controlling the composition of the cementitious binder within certain specified limits. Our cementitious binders find use in concrete or mortar for chambers and conduits of desalination plants, for similar applications in the pro- 5 duction of salt from brines, for deep oil well cementing and for other similar applications.

During our research, we found that conventional concrete formulations suffered serious strength losses, ex hibited substantial expansion and displayed severe corrosion when exposed to saline solutions at moderately ele vated temperatures. The cause of these effects was traced to the hydrothermal synthesis of the mineral ellestadite. Ellest dite is the phosphate-free end member of the isomwpfm'apatite series having an ideal composition of:

9CaO.3SiO .3SO .Ca(F ,(OH) It is slightly soluble in water and hydrolyzes giving a pH of about 9.5 at 25 C.

It is unstable and decomposes at lower pI-I'values.

In view of these properties, it would at first appear that ellectadite could not form in concrete exposed to saline Pounds Within the Cement hinder Paste Expansion Caused solutions since the pH of brines such as sea water is much y Sueh crystallization results in cracking, Spelling and lower than 9.5. However, we were able to determine that Progressive deterioration of the concrete- The most ellestadite did form beneath the surface of concrete where Portflht Sulfate-hearing Product is Cement pastes at pH remained sufficiently high and where sulfate ions from bient temperatures is calcium aluminate trisulfate known the brine were bl to migrate Expansion caused by h as ettl'ihgite- This compound forms in y cement bind growth of ellestadite crystals caused progressive rupturing at room temperature from Sulfate absorbed from and disintegration of the concrete specimens. In all of our the sill'rouhdihgsexperimental work, ellestadite was identified and char- Formation of ettringite probably causes most of the acterized b i X. diff i tt sulfate COI'I'OSIOI]. observed at normal concrete use tem- 40 After having determined the cause of concrete expanperatures. However, ettringite IS not stable at elevated ign and corrosion at elevated temperatures to be the temperatures 50 it Cannot be responsible for Sulfate formation of ellestadite, we then attempted its synthesis. rosion observed at such conditions. It has been reported A number f diff r t binder materials were prepared that ettringite does not form in cement pastes cured at i i of bl d f T 1 or T V 1 d cement i'egardiess of the Sulfate cohteht ("P to 165%)- with varying amounts of fly ash, electric furnace slags Trace amounts were present in cement pastes cured for d gypsum h bl d as pastes h i water/Cement 2 h s at Heflce, it can he Concluded that the ratios of 0.25, were cast as cubes and were cured at standmaxirnum temperature at which ettringite is stable or can d temperatures f 99, 121 d 143 for 7 d form is ab ut 90 C. Samples of the cubes were subjected to strength tests and Thus, measures taken to Prevent Sulfate attack at room were then crushed, Washed with acetone and thoroughly temperature do not necessarily pp y to moderately degassed. These dried specimens were then again crushed vated temperatures, within the range of about 90 to 150 t pass a 50 1 ree and were examined by X-ray C. yet use of concrete in contact with sulfate-containing diff ti n nd diff r ntial thermal anal i bl'ines in that temperature range WOUid he highly At the two higher temperatures (121 and 143 C.), vantageous in certain applications such as evaporator ellestadite was observed to develop in some of the binder cham rs in d salinati n processes, cements for p oil formulations. Those formulations in which ellestadite had wells and the like. developed also showed abrupt and generally large strength decreases. Both effects (ellectadite formation and strength SUMMARY OF THE INVENTION decrease) in turn were found to be dependent upon sul- We have now found that the major cause of saline cor- 6 fate content of the binder paste. Even a slight excess of rosion in cement binder pastes at moderately elevated sulfate beyond a certain optimum amount produced striktemperatures is the hydrothermal formation of a sulfateing dilferences in strength. Below this optimum sulfate bearing apatite mineral known as ellestadite. 'We have content, ellestadite did not form; above that level, ellestaalso found that formation of this mineral can be prevented dite did form. The optimum, or critical, sulfate content by controlling the composition of the cement binder withappears to be set by the amount of sulfate which will subin particular ranges of original sulfate content. Thus we have developed a concrete formulation procedure which makes practical the use of concrete as a structural material in contact with high concentrations of sulfate ion at moderately elevated temperatures.

Hence it is an object of our invention to prevent saline corrosion of concrete at elevated temperatures.

stitute into the lattice of the hydrated calcium silicate. If sulfate in excess of that amount is present during the curing process, then ellestadite is formed.

Maximum sulfate content which can be tolerated in o a binder depends upon the formulation. Tests for optimum cement-30% fly ash, 1.4% S and 50%\ Type V cement--50% ground slag, 1.0% S0 In another series of experiments a number of different binder formulations were prepared and formed into 2 by 4 inch paste cylinders. These cylinders were cured at 121 C. and then exposed to two-fold concentrated sea water (77,000 p.p.m. salts) at 121 C. for 241 days. Six different compositions were selected for detailed analysis. L the turnings were gathered at selected depths beneath the surface of the cylinders for X-ray diffraction analysis in order to determine ellestadite concentration. These data are presented in tabular form as follows:

TABLE I Original Maximum Depth of Sulfate ellestadite ellestadite maximum content coneentraconcentraellestadite (percent tion (counts Expansion, tion (counts concentra- Sample No. Binder formulation S03) per second) percent per second) tion (inch) 70% cement, 30% fly ash 1. 4 O 0.029 2 50% cement, 50% slag. 1.0 0 .022 3 Type V cement 2. 0 53 070 4 H Type I cement 2.0 53 .068 .5 TypeVcement 3.5 109 .120 6 Type I cement 3. 5 105 100 Ellestadite concentration is given in relative units in nents to a level below which ellestadite will not form; counts per second of the 2.82 A. line usmg an X-ray difand fractometer. Expansion was determined by use of 1 x 1 x (c) curing the paste at a temperature above about 90 11 inch reference prisms. In all cases, ellestadite content C. to produce a cementitious binder which is charwas zero at the surface of the test specimens in contact acterized by resistance to corrosion from sulfatewith the sulfate-containing brine. containing waters at temperatures above 90 C.

Results of these tests clearly show the expansive nature of ellestadite. More importantly, these results show that if the sulfate content of a cementitious binder is kept below a level at which ellestadite does not form during curing, then ellestadite will not form upon lengthy exposure to high concentrations of sulfate ion at elevated temperatures. Conversely, if the sulfate content of the binder is sufiiciently high to allow ellestadite formation during curing, then exposure to a sulfate brine results in additional ellestadite growth. This causes expansion of Y the binder, substantially lowered strength, and progressive deterioration'of the binder material.

Those compositions in Table I in which ellestadite did not form are illustrative of binders resistant to saline attack at elevated temperatures. Other proportions of cement of various types, preferably in admixture with slag, fly ash or other pozzolans yield comparably satisfactory results, provided of course that the sulfate content is controlled so as to prevent ellestadite formation. Generally, cements without pozzolans but of low sulfate contents, within the range of about 0.5 to 1.5%, may be used but are less desired. Because of the inversion of hydrous calcium silicate to alpha dicalcium silicate hydrate displayed by such cements, a binder of low strength having free calcium hy- 2. The method of claim 1 wherein the finely divided pouolan is chosen from the group consisting of fly ash and slag.

3. The process of claim 2 wherein the cementitious binder is associated with aggregate to form a concrete.

4. The purpose of claim 3 wherein the cementitious binder comprises about 50% cement and about 50% slag and has a sulfate content of about 1%.

References Cited UNITED STATES PATENTS 2,880,100 3/1959 Ulfstedt l06--89 2,083,179 6/1937 Work 106100 3,194,673 7/1965 Schedel 106100 OTHER REFERENCES Lea & Desch: The Chemistry of Cement and Concrete, Edward Arnold (Publishers) Ltd., London, 1956, pp. 15, 153.

JAMES E. POER, Primary Examiner US Cl. X.R. 106-97, 117 

